The invention relates to the pretreatment by electrochemical oxidation of strip or foil of aluminum, for lacquering, adhesive coating or laminating processes.
Lacquered strips or foils made of aluminum are employed for the manufacture of foodstuff packaging materials, the lacquer providing corrosion protection for the metal. For many packaging purposes it suffices for the lacquer to be deposited on the untreated aluminum. For more demanding forms of packaging, such as deep drawn containers for pasteurized or sterilized goods, however, it is necessary to pretreat the aluminum in order to achieve adequate adhesion of the lacquer.
It is known to pretreat of aluminum strip or foil whereby chemical conversion of the aluminum takes place. Usually such reactions are carried out in baths containing chromic acid, fluoric acid or phosphoric acid. When chromic acid is used for this purpose, the newly formed layer contains hexavalent chromium compounds which are increasingly regarded as unacceptable in foodstuff packaging. In view of this, non-electrolytic methods for producing Cr.sup.VI -free protective coatings have already been developed. The quality of such coatings is, however, inadequate with respect to both lacquer adhesion and corrosion resistance especially for sterilizable forms of packaging.
German Pat. DE-PS No. 17 71 057 reveals a continuous pretreatment process for aluminum strip prior to coating the same with lacquer wherein the strip is passed through an aqueous, sulphuric acid bath, which is at a temperature of 80.degree.-100.degree. C. and features electrodes which enable the strip to be anodized under the application of alternating current. The facilities for carrying out this process comprise two counter electrodes which are separate and are insulated from the bath. These electrodes are connected to an alternating current source and are immersed in the sulphuric acid electrolyte. The metal strip passing through the electrolyte is given, by the electrodes and corresponding to the frequency of the alternating current, a positive and a negative potential. This has the effect of causing an oxide layer to form on the aluminum strip, and for the greater part to redissolve again when the aluminum strip becomes the cathode. As such the dissolution of the approximately 0.05 .mu.m thick oxide layer does not occur uniformly but locally so that craters are formed. These craters provide a suitable roughened base for adhesion of the subsequently deposited lacquer.
The object of the present invention is to develop a process which provides adequate adhesion for subsequently deposited lacquer, adhesive or laminating material, at least for the normal, present day requirements, and such that the surface layer formed on the aluminum is free of heavy metals. Furthermore the pretreatment should be continuous, and faster than the conventional process.